Move reusable code from mobile to common

This will be useful for a future desktop application.

mobile/lib/main.dart

@@ -1,12 +1,26 @@
 import 'package:audio_service/audio_service.dart';
 import 'package:flutter/widgets.dart';
+import 'package:musicus_common/musicus_common.dart';
+import 'package:path/path.dart' as p;
+import 'package:path_provider/path_provider.dart' as pp;
 
 import 'app.dart';
-import 'backend.dart';
+import 'settings.dart';
+import 'platform.dart';
+import 'playback.dart';
+
+Future<void> main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+
+  final dir = await pp.getApplicationDocumentsDirectory();
+  final dbPath = p.join(dir.path, 'db.sqlite');
 
-void main() {
   runApp(AudioServiceWidget(
-    child: Backend(
+    child: MusicusBackend(
+      dbPath: dbPath,
+      settingsStorage: SettingsStorage(),
+      platform: MusicusAndroidPlatform(),
+      playback: Playback(),
       child: App(),
     ),
   ));